A diesel particulate filter (hereinafter referred to as “DPF”), for example, is known as a filter for collecting particulate matter (PM) contained in an exhaust gas emitted from a diesel engine.
The DPF can only collect a limited amount of PM, and therefore a so-called forced regeneration for periodically burning and removing accumulated PM needs to be carried out. The forced regeneration is carried out by supplying unburned fuel (HC) to an oxidation catalyst on an upstream exhaust pipe through in-pipe injection or post-injection, and raising the temperature of the exhaust gas to a PM-burning temperature with the heat produced upon oxidation.
The PM that accumulates in the DPF contains a soot component and so on emitted primarily from an engine. An engine oil component and so on also accumulates in the DPF. Among these components, the soot component is burned and removed by the forced regeneration, but the engine oil results in residual ash. The ash component is incombustible and cannot be removed even if the forced regeneration is carried out. As the ash component further accumulates, the DPF becomes clogged with the ash component. This brings about an increased pressure loss. To deal with this, maintenance for periodically cleaning the accumulated ash becomes necessary.
There is known a technique in which the timing for maintaining a DPF is predicted by estimating the amount of accumulated ash from a pressure difference across the DPF. The pressure difference across the DPF is detected immediately after the forced regeneration of the DPF finishes (e.g., see PATENT LITERATURE 1).